Dissertations / Theses on the topic 'Low velocity zone'

Seismic studies over the past decade have identified a S-wave low-velocity zone (LVZ) above the transition zone at various locations around the globe. This layer is hypothesized to be a lens of dense, fluid-rich silicate melt ponding atop the 410 km discontinuity, beneath the silicate melt-density crossover predicted to exist within the upper mantle. We have assembled a P- and S-receiver function (PRF and SRF, respectively) dataset from the CNSN Yellowknife Array (YKA), the CANOE array, and the POLARIS-Slave array, to quantify the physical properties and geographical extent of the layer in Northwestern Canada. In order to compute the Poisson's ratio, an important discriminant of possible composition and/or fluid content, we generated a suite of 1-D velocity models based on IASP91, but with varying thicknesses and velocity ratios for a hypothetical layer above the 410 km discontinuity. From these models we computed moveout curves for the range of slowness represented in the YKA data. A grid search was performed over the model space of interval thickness and Poisson's ratio to obtain an estimate of the model that best accounts for the data. In addition, we performed a linearized inversion of transmission coefficient amplitudes to estimate the shear velocity contrast at the bounding interfaces of the LVZ. Results indicate a LVZ of thickness 36 km with a shear velocity contrast of -7.8%, and Poisson's ratio of 0.42. In combination, these two results require an associated increase in compressional velocity into the LVZ. The Poisson's ratio lies well above the IASP91 average of 0.29-0.3 for this depth range and favours the presence of high melt or fluid fractions. Geographic profiles of PRFs and SRFs 1-D migrated to depth from CANOE and POLARIS-Slave arrays reveal 410 km and 660 km discontinuities at nominal depths with little variation in transition zone thickness. PRF results from the Slave craton indicate a potential LVZ beneath many stations at an average nominal depth of 340 km, highlighted by events from the northwest. The CANOE array SRF profile images an emergent LVZ beginning at 280 km depth dipping eastwards to 310 km approaching YKA.

The effects of lithology, fracturing, and gouge zone mineralization on the geophysical properties of fault zones are not very well understood. In situ seismic data collected over the exhumed San Gregorio Fault at Moss Beach, CA were used to relate in situ compressional wave velocity to internal fault zone properties. This active strike-slip fault is exposed in cross section on an uplifting and actively eroding wave-cut platform. It cuts shallow marine sediments that have been buried to depths of a few kilometers. The unweathered exposure containing seawater makes it a unique analog of subsurface faults. Previous structural analysis over this exposure observed damage caused by faulting over a ~100 m wide zone in cross-section. The fault zone is centered at a 10-17 m wide clay-rich fault core flanked by a ~30 m wide brecciated gouge zone. These gouge zones are bordered on either side by 30-40 m wide fractured zones. Resolving to a scale of a few meters, the seismic survey produced a continuous P-wave velocity profile analogous to a horizontal well log across the fault. Lateral variations in the velocity profile correlate exactly to previously mapped fault zone structure. The clay core and adjacent brecciated gouge create a ~50 m wide very low velocity zone, 25-50% slower than the surrounding host rock. Fractured bedrock on either side of the core causes a wider zone of 5-10% slow velocity, for a total fault signature ~100 m wide. Fault parallel fracture anisotropy was observed in the fractured zones, but surprizingly anisotropy was not observed in the strongly foliated gouge zones. The field measurements differ significantly from laboratory measurements at zero pressure and in some cases from expected values for saturated rock of this porosity, perhaps due to biased rock sampling, the long wavelength effects of macrofractures, frequency dispersion, and partial saturation. The velocity profile is similar in width and consistent in velocity contrast to low S-wave velocity zones derived from fault zone guided waves in other strike-slip faults. The traveltime delay across the fault zone is not large enough to cause the 2-3 km wide crustal low velocity zones modeled by refraction studies. Synthetic reflection seismograms in the typical frequency range show that the fault zone acts as a thick bed or as a constructively interfering thin bed. The models suggest that very large reflection coefficients observed across accretionary prism faults can be explained by fracturing, brecciation and clay content without elevated pore pressures. Comparison with a refraction study across the Punchbowl Fault shows a similar structural zonation of these two well-studied examples of brittle fault zones. This suggests that high-resolution seismic velocity models can be used to directly interpret internal deformation structure of brittle faults.
Master of Science

Dipping low-velocity zones (LVZs) are a ubiquitous structural element of subduction zones worldwide. In this study we map seismic attributes characterizing the LVZ beneath the Cascadia subduction zone from northern Vancouver Island to northern California using receiver function waveform inversion. Throughout this region, the LVZ is characterized by high Vp/Vs ratios (mean=2.77), strong S-velocity contrasts (~50%) and thicknesses averaging 3.38 km. The LVZ is immediately underlain by a second, weaker layer exhibiting moderate Vp/Vs ratios (mean=1.85) with mean thickness of 4.62 km. We interpret the combined structure in terms of subducting oceanic crust, based on classical structural/petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores. The LVZ is identified with pervasively hydrated, high porosity pillow basalts and sheeted dikes of Layer 2 with possible contributions from sediments (Layer 1). Fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above, and a low porosity, gabbroic/mafic-cumulate dominated Layer 3 below.

A new high-resolution 3-dimensional P-wave velocity model for Mt. Pinatubo volcano was developed by tomographic inversion of P-wave arrivals from 3,007 earthquakes recorded during a four month period from May to August, 1991. The arrivals were recorded by a network of seismic stations, consisting of seven pre-eruption stations and seven post-eruption stations. Two stations survived the June eruptions. First-arrival travel times were calculated using a finite-difference solution to the eikonal equation. An iterative, linearized approximation of the nonlinear tomography problem was used to solve separately for both velocity structure and hypocenter locations. Several inversions performed with different initial parameters and convergence schemes, and synthetic checkerboard reconstructions indicate a horizontal spatial resolution of velocity perturbations near 4 km. However, the network sparseness allows for a substantial trade-off between focal depth, origin time, and the vertical velocity profile. Many hypocenter clusters collapse from diffuse clouds into tighter features after 3-D relocation. These bands of earthquakes appear to represent fault-related structures. Three low-velocity (relative to the horizontal average) anomalies exist within the well-resolved portion of the velocity model. These anomalies are spatially associated with pre- and post-eruption earthquakes oriented along mapped surface fault zones. Similar anomalies observed at different volcanoes have been previously interpreted as magma related. The low-velocity anomalies at Pinatubo are interpreted as highly fractured, hot volumes of mostly competent rock, which may contain partial melt.
Master of Science

The diffusion of composite laminates in aerospace industry has been slowed down by complexity in the prediction of fracture behaviours. In this respect the delamination phenomenon caused by Low-Velocity Impacts has been a critical issue. Several criteria that predict the delamination onset and growth have been analysed. The subsequent study has been focused on Cohesive Zone Models able to predict both initiation and propagation of delamination. Several models that represent the dynamic response of composite structures to impacts have been presented. An explicit FEM has been developed to perform 3D simulations of different layup configurations of Al2024T3 and Woven Carbon Prepreg Laminates subjected to a Low-Velocity Impact. ABAQUS, Dassault Systèmes Simulia Corp. has been employed to perform the numerical simulations. Specific attention is paid to the cohesive failure representing delamination.

The structure of fault zones (FZs) plays an important role in understanding fault mechanics, earthquake rupture and seismic hazards. Fault zone seismic guided waves (GW) carry important information about internal structure of the low-velocity fault damage zone. Numerical modeling of observed FZGWs has been used to construct models of FZ structure. However, the depth extent of the waveguide and the uniqueness of deep structure in the models have been debated. Elastic finite-difference synthetic seismograms were generated for FZ models that include an increase in seismic velocity with depth both inside and outside the FZ. Strong GWs were created from sources both in and out of the waveguide, in contrast with previous homogenous-FZ studies that required an in-fault source to create GW. This is because the frequency-dependent trapping efficiency of the waveguide changes with depth. The near-surface fault structure efficiently guides waves at lower frequencies than the deeper fault. Fault structure at seismogenic depth requires the analysis of data at higher frequencies than the GWs that dominate at the surface. Adapting a two-station technique from surface wave studies, dispersive differential group arrival times between two earthquakes can be used to solve for FZ structures between the earthquakes. This method was tested with synthetic data and shallow events recorded in the SAFOD borehole in the San Andreas Fault. A pair of deep earthquakes recorded in the SAFOD borehole indicate a ~150 m wide San Andreas Fault waveguide with >20% velocity contrast at 10-12 km depth. With additional earthquakes, the full FZ structure at seismogenic depth could be imaged. Subsurface FZ structure can also be derived from a surface source and receiver array analogous to a body-wave refraction survey. Synthetic seismograms for such source-receiver geometry were generated and verified that FZGWs are refracted by the increase in velocity with depth. Synthetic data from a surface array were successfully inverted to derive FZ structure in the subsurface. The new methods presented in this dissertation extend the potential of FZGWs to image deeper FZ structure than has been uniquely constrained in the past.
Ph. D.

Ce travail s’inscrit dans la problématique de réduction de masse, de sécurité inhérent au domaine aéronautique, il concerne plus spécifiquement les sièges de pilotes d’avion de ligne. Un nouveau concept d’assise composite sandwich multifonctionnel est proposé. Il est composé d’une peau carbone, d’une âme nid d’abeille Nomex et d’une peau hybride Kevlar/lin. L’assemblage de plusieurs matériaux engendre des comportements parfois complexes et rend difficile la prédiction de la ruine de la structure. Une démarche expérimental/numérique est mise en place pour appréhender l’endommagement de l’assise et ainsi permettre un pré-dimensionnement via un outil numérique.Tout d’abord, des essais de caractérisation permettent d’élaborer les lois de comportement des différents matériaux constituant le sandwich. Le composite hybride présente un comportement élasto-plastique-endommageable-anisotrope. Le nida Nomex est représenté par un réseau de ressort et une loi couplant le comportement en compression et en cisaillement qui est implémentée dans ABAQUS. Des essais d’impacts permettent d’évaluer les modes de rupture et l’énergie dissipée par les concepts d’assises réalisés. Des simulations numériques intégrant les comportements matériaux identifiés sont mises en places pour corréler l’essai d’impact. L’analyse couplée des résultats expérimentaux et numériques permet d’identifier les couplages entre les différents mécanismes. Enfin, le modèle est utilisé pour dimensionner une assise composite qui s’avère sans optimisation fine, comparable à une assise existante en aluminium de l’A350
This work is part of the problem of mass reduction, safety inherent in the aeronautical field, it concerns more specifically the seats of pilots of airliner. A new multi-functional sandwich composite seat pan is proposed, composed by a carbon skin, a Nomex honeycomb core and a Kevlar/flax hybrid skin. The assembly of several materials generates complex behaviors and makes the ruin of the structure difficult to predict. An experimental/numerical approach is used to understand the damage mechanism of the seat and to create a pre-dimensioning numerical tool.Firstly, characterization tests allow identifying the mechanical behaviors of each material and constituting a database for the creation of material laws. The hybrid composite shows an elastoplastic-damaging-anisotropic behavior. The honeycomb is represented by a spring network and a law coupling the compression and shear behavior is implemented. Impact tests are used to evaluate the failure modes and the energy dissipated by the different concepts. The impact tests are correlates by numerical simulation using the identified material behaviors. The analysis of the experimental and numerical results makes it possible to identify the coupling between the different mechanisms. Finally, the model is used to design a new composite seat pan. This one is comparable to the existing aluminum seat pan without optimization phase

La fusion partielle dans la partie supérieure du manteau est fréquente dans les zones de remontée d’eau du manteau, comme les rifts, les dorsales médio-océaniques et les points chauds. Des zones de faible vitesse d’ondes de cisaillement dans le manteau peu profond (∼ 80 km) sont souvent associées à ces endroits, révélant des anomalies de vitesse de 4 à 5 % généralement attribuées à la présence de matériau fondu. Cependant, les études sur la quantité de matière fondue responsable de la réduction de la vitesse conduisent, suivant leur nature, à des conclusions différentes: les résultats expérimentaux de la pétrologie, des observations géochimiques et des modèles géodynamiques suggèrent une rétention de la matière fondue inférieure à 1 %, alors que les interprétations sismiques exigent une rétention supérieure à 1%. Dans cette thèse, j’essaie de résoudre le désaccord sur la rétention de la masse fondue dans l’asthénosphère en croisant des modèles de production de matériau en fusion et de propagation des ondes sismiques pour relier directement les conditions géodynamiques de la fusion partielle aux observations sismiques du panache mantellique de la Réunion. J’ai mis au point un modèle 1D de production de fusion qui évalue la rétention de la matière fondue selon le coefficient de perméabilité, la températures initiale du manteau et la vitesses de remontée d’eau selon un problème de Stokes modifié supposant un écoulement poreux. 210 scénarios de modèles de fusion sont convertis en vitesses d’ondes P et S sismiques anharmoniques à l’aide d’une base de données de paramètres minéraux, qui sont ensuite incorporés dans le modèle de référence de la terre ak135 pour générer des sismogrammes synthétiques des scénarios de fusion pour 21 tremblements de Terre. J’analyse l’effet du matériau fondu sur les composantes radiale, transversale et verticale de l’onde pour les arrivées de phase P, S, Pdiff et SKS. Grâce à une procédure automatisée de corrélation croisée, je calcule les différences relatives de temps de parcours entre les sismogrammes observés et les 210 traces synthétiques, pour chacun des 21 événements, 4 arrivées de phase, 3 composantes d’onde et 4 fréquences différentes de filtrage. J’analyse 70 896 points de données de temps de parcours relatifs pour trouver une solution optimale rendant compte des temps de parcours relatifs entre les traces du modèle et les observations sismiques, afin de découvrir quel scénario de fusion décrit le manteau supérieur sous la Réunion. La solution au scénario du modèle le mieux adapté n’est pas unique, puisque plusieurs combinaisons du coefficient de perméabilité, de température et de vitesse de remontée peuvent donner la même solution. En analysant séparément la distribution des paramètres du modèle sur la solution de temps de parcours relatif minimisé des 70 896 points de données pour les différentes arrivées de phase et composantes des ondes, on peut identifier deux régimes probables des conditions du manteau supérieur pouvant rendre compte des observations sismiques. Les conditions de manteau sous la Réunion se situent soit dans la plage de température de 1300 à 1350°C avec des fractions en fusion de ∼ 1 %, soit dans la plage de température de 1400 à 1450 °C avec des fractions en fusion inférieures à 0,3 %. Les contraintes des études sur la température du manteau supérieur, la perméabilité et les vitesses de transport du matériau fondu correspondent à ce dernier cas, ce qui souligne qu’une faible rétention de matériau fondu dans le manteau peu profond sous la Réunion satisfait simultanément les observations sismiques et les conditions géodynamiques prévues
Partial melting in the upper mantle is prevalent in areas of mantle upwelling such as rifts, mid-ocean ridges and hotspots. The presence of low shear-wave velocity zones in the shallow mantle (∼ 80km) are often associated with these locations, revealing velocity anomalies of −4 to −5% which are generally attributed to the presence of melt. However, studies on the quantity of melt responsible for the velocity reduction are divided, where experimental results from petrology, geochemical observations and geodynamical models suggest melt retention of < 1%, whereas seismic interpretations call for 1 > %. In this thesis I attempt to resolve the disagreement on melt retention in the asthenosphere by combining forward modelling of melt production and seismic wave propagation to relate geodynamic condition of partial melting directly to seismic observations of the Réunion mantle plume. I developed a 1D model of melt production that approximates melt retention for a range of permeability coefficients, initial mantle temperatures and upwelling velocities through a set of modified Stokes equations assuming porous flow. 210 melting model scenarios are converted to anharmonic seismic P- and S-wave velocities using a mineral parameter database, which are embedded into the ak135 earth reference model to generate synthetic seismograms of the melting scenarios for 21 source events using the Direct Solution Method for a laterally homogeneous and spherically symmetrical Earth. I explore the effect of melt presence on the radial, transverse and vertical wave component for the P, S, Pdiff and SKS phase arrivals, band-pass filtered to upper corner frequencies of 0.05, 0.1, 0.15 and 0.2 Hz. Through an automated cross-correlation procedure I compute relative traveltime differences between the observed seismograms and the 210 synthetic model traces for each iteration of the 21 source events, 4 phase arrivals, 3 wave components, and 4 band-pass filter frequencies. I analyse 70,896 relative traveltime datapoints to reach a solution for the minimised relative traveltimes between the model traces and the seismic observations, in order to discover which melting model scenario describes the upper mantle beneath Réunion. The solution to the best-fit model scenario is non-unique, since several combinations of the permeability coefficient, temperature and upwelling velocity give the same solution. By seperately analysing the parameter distribution of the free model parameters over the minimised relative traveltime solution of the 70,896 datapoints for the different phase arrivals and wave components, two likely regimes of upper mantle conditions can be constrained that can resolve the seismic observations. These regimes indicate that mantle conditions beneath Réunion are either in the 1300−1350 °C temperature range with melt fractions of ∼ 1%, or in the 1400−1450 °C temperature range with melt fractions of < 0.3%. Constraints from studies on upper mantle temperature, permeability and melt transportation velocities correspond to the latter case, showing that low retention of melt in the shallow mantle beneath Réunion simultaneously satisfy seismic observations and the expected geodynamic conditions

Seismic data from the intermediate-spreading Endeavour segment of the Juan de Fuca Ridge reveal several crustal-level, low-velocity, high-attenuation regions on the eastern and western ridge flanks 7 to 16 km from the neovolcanic zone. I examine Pg amplitude anomalies for a wide variety of source-receiver azimuths in the Endeavour active source seismic tomography data. I use finite difference waveform forward modeling to estimate the dimensions, depth, and seismic properties of the best-observed inferred anomalous regions. The attenuating regions extend 10-15 km beneath axis-parallel bathymetric highs and from 2 to 4 km below the seafloor. The velocity reduction is small (~8%) and the attenuation large (QP ≈ 8-40) suggesting the presence of partial melt. I infer that melt focusing toward the neovolcanic zone is incomplete and that tectonic interactions with the Heckle seamount chain and/or the large segment-bounding overlapping spreading centers may promote off-axis melt delivery at the Endeavour segment.

Les serpentines, résultant de l'hydratation des minéraux mantelliques, pourraient être acteur majeur dans le transport de l'eau dans le manteau terrestre. Ayant une faible viscosité elle pourraient jouer un rôle déterminant dans la déformation et la répartition des contraintes dans les zones de subduction. Les équations d'état P-V des trois principales variétés de serpentines, lizardite, chrysotile et antigorite ont été obtenues expérimentalement. Une loi de déformation a été extraite d'expériences de déformation sur l'antigorite à HP-HT (200°C-500°C et 1à 4 GPa) et faible vitesse de déformation , réalisées sur synchroton avec une presse D-DiA. Les résultats expérimentaux permettent de discuter i) de la dynamique du coin de manteau dans la zone de subduction, ii) du rôle des serpentines dans la sismicité, et la relaxation inter-sismique et post-sismique.

Nous avons étudié des magmas ferrifères silicatés magnésiens à la pression du manteau terrestre en utilisant la dynamique moléculaire (First Principles Molecular Dynamics). Les résultats de l’équation d’état que nous avons obtenus à partir de nos simulations ont été utilisés pour créer un modèle chimique et minéralogique pour les zones de très basse vitesse sismique (ULVZ, anomalies régionales dans le manteau proche de la limite noyau-manteau). De plus, nous avons étudié le comportement du Ni, du Co et du Fe dans ces magmas et établi la dépendance du spin en fonction de la concentration, de la pression, de la température et du degré de polymérisation du magma silicaté. Nous avons montré qu’une baisse du spin moyen peut être corrélée au changement de pente (kink) observé précédemment pour les coefficients de partage du Ni et du Co. Nous avons analysé la structure du magma pour toutes les compositions étudiées en fonction de la pression. Nos résultats donnent un nouvel aperçu de la coordination des éléments majeurs et traces dans les magmas silicatés de différents degrés de polymérisation. Nous interprétons l’anomalie de coordination Ni-O en fonction de la pression comme un changement d’état de spin. L’effet de la polymérisation du magma silicaté sur les coefficients de partage du Co, du Ni et du W entre le métal et le magma silicaté a été étudié par expériences multi-enclumes en conditions isobares et isothermes. Nous avons réalisé des simulations FPMD de magmas à des degrés de polymérisation similaires aux expériences afin d’expliquer le caractère de plus en plus lithophile du W lorsque le degré de polymérisation du magma silicaté diminue. Nous proposons une explication structurale pour expliquer l’affinité décroissante apparente du W dans les magmas silicatés dépolymérisés
We explore Fe-bearing Mg-silicate melts through the pressure regime of the Earth’s mantle using First Principles Molecular Dynamics (FPMD). The equation of state results we obtained from our simulations are used to create a chemical and mineralogical model for Ultra-Low Velocity Zones (anomalous region on the mantle side of the core-mantle boundary). Furthermore we study the behaviour of Ni, Co, and Fe in these melts, and asses their spin-crossover dependencies on their concentration, pressure, temperature, and the degree of polymerization of the silicate melts. We show that a decrease in the average spin can be correlated with the previously observed kink in the partitioning coefficient of Ni and Co. We investigate the melt structure of all the compositions studied as a function of pressure. Our results provide new insight into the coordination of major and trace elements in silicate melts with different degrees of polymerization. We interpret the anomalous Ni-O coordination trend with pressure as the result of the spin state change. The effect of silicate melt polymerization on the partitioning of Co, Ni, and W between a metal and silicate melt, is investigated at isobaric and isothermic conditions using multi-anvil experiments. We have performed FPMD simulations of melts with similar degrees of polymerization as the experiments in order to explain the increasing lithophile character of W with the decrease in polymerization of the silicate melt. We propose a structural explanation for tungsten’s apparent increased affinity for depolymerized silicate melts

Assuming that liquid iron alloy from the outer core interacts with the solid silicate-rich lower mantle the influence on the core-mantle reflected phase PcP is studied. If the core-mantle boundary is not a sharp discontinuity, this becomes apparent in the waveform and amplitude of PcP. Iron-silicate mixing would lead to regions of partial melting with higher density which in turn reduces the velocity of seismic waves. On the basis of the calculation and interpretation of short-period synthetic seismograms, using the reflectivity and Gauss Beam method, a model space is evaluated for these ultra-low velocity zones (ULVZs). The aim of this thesis is to analyse the behaviour of PcP between 10° and 40° source distance for such models using different velocity and density configurations. Furthermore, the resolution limits of seismic data are discussed. The influence of the assumed layer thickness, dominant source frequency and ULVZ topography are analysed. The Gräfenberg and NORSAR arrays are then used to investigate PcP from deep earthquakes and nuclear explosions. The seismic resolution of an ULVZ is limited both for velocity and density contrasts and layer thicknesses. Even a very thin global core-mantle transition zone (CMTZ), rather than a discrete boundary and also with strong impedance contrasts, seems possible: If no precursor is observable but the PcP_model /PcP_smooth amplitude reduction amounts to more than 10%, a very thin ULVZ of 5 km with a first-order discontinuity may exist. Otherwise, if amplitude reductions of less than 10% are obtained, this could indicate either a moderate, thin ULVZ or a gradient mantle-side CMTZ. Synthetic computations reveal notable amplitude variations as function of the distance and the impedance contrasts. Thereby a primary density effect in the very steep-angle range and a pronounced velocity dependency in the wide-angle region can be predicted. In view of the modelled findings, there is evidence for a 10 to 13.5 km thick ULVZ 600 km south-eastern of Moscow with a NW-SE extension of about 450 km. Here a single specific assumption about the velocity and density anomaly is not possible. This is in agreement with the synthetic results in which several models create similar amplitude-waveform characteristics. For example, a ULVZ model with contrasts of -5% VP
-15% VS and +5% density explain the measured PcP amplitudes. Moreover, below SW Finland and NNW of the Caspian Sea a CMB topography can be assumed. The amplitude measurements indicate a wavelength of 200 km and a height of 1 km topography, previously also shown in the study by Kampfmann and Müller (1989). Better constraints might be provided by a joined analysis of seismological data, mineralogical experiments and geodynamic modelling.
Unter der Annahme, dass flüssiges Eisen aus dem äußeren Erdkern mit dem festen, silikat-reichen Unteren Mantel reagiert, wird eine Einflussnahme auf die Kern-Mantel Reflexionsphase PcP erwartet. Ist die Kern-Mantel Grenze aufgeweicht, und nicht wie bislang angenommen ein diskreter Übergang, so zeichnet sich dies in der Wellenform und Amplitude von PcP ab. Die Interaktion mit Eisen führt zu teilweise aufgeschmolzenen Bereichen höherer Dichte, welche die seismischen Wellengeschwindigkeiten herabsetzen. Basierend auf den Berechnungen von kurzperiodischen synthetischen Seismogrammen, mittels der Reflektivitäts- und Gauss Beam Methode, soll ein möglicher Modellraum dieser Niedriggeschwindigkeitszonen ermittelt werden. Das Ziel dieser Arbeit ist es das Verhalten von PcP im Distanzbereich von 10° bis 40° unter dem Einfluss dieser Modelle mit diversen Geschwindigkeits- und Dichtekontrasten zu untersuchen. Ferner wird das Auflösungsvermögen hinsichtlich seismischer Daten diskutiert. Entscheidende Parameter wie Anomaliedicke, Quellfrequenz und Topographie werden hierbei analysiert. Tiefe Erdbeben und Kernexplosionen, die sich im entsprechenden Entfernungsbereich zum Gräfenberg und NORSAR Array befinden, werden anschließend im Hinblick auf PcP ausgewertet. Das seismische Auflösungsvermögen von Niedriggeschwindigkeitszonen ist stark begrenzt sowohl in Bezug auf Geschwindigkeits- und Dichtekontraste als auch hinsichtlich der Mächtigkeit. Es besteht sogar die Möglichkeit einer dünnen, globalen Kern-Mantel Übergangszone, selbst mit großen Impedanzkontrasten, ohne dass dies mit seismologischen Methoden detektiert werden könnte: Wird kein precursor zu PcP beobachtet aber das PcPmodel /PcPsmooth Amplitudenverhältnis zeigt gleichzeitig eine Reduktion von mehr als 10%, dann könnte eine sehr dünne Niedriggeschwindigkeitszone von ca. 5 km Mächtigkeit und einer Diskontinuität erster Ordnung vorliegen. Andererseits, ist PcP um weniger als 10% reduziert, könnte dies entweder auf eine dünne, moderate Niedriggeschwindigkeitszone oder einen graduellen Kern-Mantel Übergang hindeuten. Die synthetischen Berechnungen ergeben starke Amplitudenvariationen als Funktion der Distanz, welche auf den Impedanzkontrast zurückzuführen sind. Dabei ergibt sich ein primärer Dichteeffekt im extremen Steilwinkelbereich und ein maßgeblicher Geschwindigkeitseinfluss im Weitwinkelbereich. Im Hinblick auf die modellierten Resultate lässt sich eine 10 - 13.5 km mächtige Niedriggeschwindigkeitszone 600 km südöstlich von Moskau mit einer NW-SE Ausdehnung von mindestens 450 km folgern, wobei eine exakte Aussage über Geschwindigkeiten und Dichte nicht möglich ist. Dies ist im Konsens mit den synthetischen Berechnungen, wonach viele unterschiedliche Modelle ähnliche Amplituden- und Wellenformcharakteristiken erzeugen. Zum Beispiel erklärt ein Modell mit Kontrasten von -5% VP
-15% VS and +5% Dichte die gemessenen PcP Amplituden. Darüber hinaus können unterhalb des südwestlichen Finnlands und nord-nordwestlich des Kaspischen Meeres Undulationen an der Kern-Mantel Grenze selbst vermutet werden. Unter Berücksichtigung früherer Studien, z. B. von Kampfmann and Müller (1989), deuten die Messergebnisse auf eine laterale Topographie von 200 km und eine Höhe von 1 km hin. Eine Eingrenzung der potentiellen Anomaliemodelle kann nur durch eine gemeinsame Auswertung mit mineralogischen Experimenten und geodynamischen Modellierungen erfolgen.

Cette thèse est une contribution à la fois expérimentale, théorique et numérique à l'étude des écoulements bidimensionnels de fluides complexes dans une conduite cylindrique présentant des singularités et dans une géométrie annulaire à cylindres excentrés. Le fluide utilisé est une solution de xanthane à différentes concentrations présentant un caractère non newtonien rhéofluidifiant. L'objectif principal de cette thèse est la caractérisation de l'influence des propriétés rhéofluidifiantes sur le comportement des zones de recirculation, en terme de morphologie, de positionnement et d'intensité, par l'utilisation et le développement de techniques de mesures non intrusives et performantes. La première méthode expérimentale utilisée une technique laser classique: la vélocimétrie par images de particules. La seconde technique mise en oeuvre est une méthode originale: la vélocimétrie par imagerie par résonance magnétique. Elle est utilisée pour la première fois au laboratoire pour la mesure de champ de vitesse d'écoulement de fluides complexes en conduite cylindrique, représentant l'intérêt majeur de cette thèse. La première partie de notre travail consiste en une description rhéologique complète de nos fluides modèles avec la détermination de leur loi de comportement et la mise en évidence de leurs propriétés viscoélastiques, par ailleurs négligeables. Par la suite les mesures de champ de vitesse des écoulements bidimensionnels étudiés et la représentation des lignes de courant montrent que les propriétés rhéofluidifiantes influencent très fortement la structure et la morphologie de ces écoulements et le comportement des zones de recirculation. Par une étude fine nous observons qu'il existe une compétition entre les effets d'inertie et les effets rhéofluidifiants induisant un champ de contrainte variable qui modifie le positionnement et la taille de la zone de recirculation. Nous montrons également que l'augmentation du caractère rhéofluidifiant affaiblit son intensité de la zone de recirculation. Enfin, des simulations numériques utilisant la loi de comportement macroscopique déterminée par rhéométrie classique ont été réalisées avec le logiciel Fluent. Une bonne concordance est observée entre les résultats de ces simulations numériques et les expérimentaux. Cette comparaison permet ainsi de valider le code de calcul et la loi de comportement, utilisée pour les simulations numériques au travers de sa modélisation suivant la loi de Cross, pour les écoulements considérés
This thesis is an experimental and numerical study of structured fluids bidimensional flows in a cylindrical pipe with singularity and in an annular geometry with eccentric cylinders. The objective of this thesis is to characterize the influence of the shear thinning properties on the recirculation zones by using efficient and non-intrusive techniques: particle image velocimetry and velocimetry by nuclear magnetic resonance imaging. Materials are xanthane solutions at different concentrations. In the first part, we determine the rheological and viscoelastic properties of the fluids used. The second part concerns the measured velocity field. It is shown that the shear thinning behavior have a strongly influence on the structure and the morphology of these flows and the pattern of the recirculation zones. Simultaneously, numerical simulations performed by Fluent and using the rheological behavior. A good concordance is observed between the experimental and numerical results. For the flows considered here, this comparison allows to validate the computational code and the behavior law used in the numerical simulations and modelling by a Cross model

abstract: The interior of Earth is stratified due to gravity. Therefore, the lateral heterogeneities observed as seismic anomalies by seismologists are extremely interesting: they hold the key to understand the composition, thermal status and evolution of the Earth. This work investigates seismic anomalies inside Earth’s lowermost mantle and focuses on patch-like ultra-low velocity zones (ULVZs) found on Earth’s core-mantle boundary (CMB). Firstly, all previous ULVZ studies are compiled and ULVZ locations on the CMB are digitized. The result is a database, which is publicly available online. A key finding is that there is not a simple mapping between the locations of the observed ULVZs and the large low velocities provinces (LLVPs). Instead, ULVZs are more likely to occur near LLVP boundaries. This spatial correlation study supports a compositionally distinct origin for at least some ULVZs. Next, the seismic structure of the basal mantle beneath the Central America is investigated. This region hosts present and past subducted slabs, which could have brought compositionally distinct oceanic basalt all the way down to the CMB. The waveform distortions of a core-reflected seismic phase and a forward modeling method are used to constrain the causes of the CMB structures. In addition to ULVZ structures, isolated patches of thin zones with shear velocity increased by over 10% relative to background mantle are found for the first time. Ultra-high velocity zones (UHVZs) are interspersed with ULVZs and could be caused by subducted mid-ocean ridge basalt (MORB) that undergoes partial melting and melt segregation. Fe-rich partial melt of MORB can form ULVZs, and silica polymorphs (SiO2) and calcium-perovskite (CaPv) rich solid residue can explain the UHVZs. Finally, large-scale heterogeneities in the lowermost mantle are investigated using S waveform broadening observations. Several basal layer models are case-studied via synthetic calculations. S wave arrivals received at a distance larger than 80˚ in a global dataset from large earthquakes between the years 1994 and 2017 are examined and S waveform broadenings are documented. This approach exploits large distance data for the first time, and therefore is complementary to previous studies in terms of sampling locations. One possible explanation of S waveform broadening is velocity discontinuity inside the D″ layer due to the temperature controlled Bm-pPv phase transition.
Dissertation/Thesis
Doctoral Dissertation Geological Sciences 2020

abstract: This research investigates Earth structure in the core-mantle boundary (CMB) region, where the solid rocky mantle meets the molten iron alloy core. At long wavelengths, the lower mantle is characterized by two nearly antipodal large low shear velocity provinces (LLSVPs), one beneath the Pacific Ocean the other beneath Africa and the southern Atlantic Ocean. However, fine-scale LLSVP structure as well as its relationship with plate tectonics, mantle convection, hotspot volcanism, and Earth's outer core remains poorly understood. The recent dramatic increase in seismic data coverage due to the EarthScope experiment presents an unprecedented opportunity to utilize large concentrated datasets of seismic data to improve resolution of lowermost mantle structures. I developed an algorithm that identifies anomalously broadened seismic waveforms to locate sharp contrasts in shear velocity properties across the margins of the LLSVP beneath the Pacific. The result suggests that a nearly vertical mantle plume underlies Hawaii that originates from a peak of a chemically distinct reservoir at the base of the mantle, some 600-900 km above the CMB. Additionally, acute horizontal Vs variations across and within the northern margin of the LLSVP beneath the central Pacific Ocean are inferred from forward modeling of differential travel times between S (and Sdiff) and SKS, and also between ScS and S. I developed a new approach to expand the geographic detection of ultra-low velocity zones (ULVZs) with a new ScS stacking approach that simultaneously utilizes the pre- and post-cursor wavefield.. Strong lateral variations in ULVZ thicknesses and properties are found across the LLSVP margins, where ULVZs are thicker and stronger within the LLSVP than outside of it, consistent with convection model predictions. Differential travel times, amplitude ratios, and waveshapes of core waves SKKS and SKS are used to investigate CMB topography and outermost core velocity structure. 1D and 2D wavefield simulations suggest that the complicated geographic distribution of observed SKKS waveform anomalies might be a result of CMB topography and a higher velocity outermost core. These combined analyses depict a lowermost mantle that is rich in fine-scale structural complexity, which advances our understanding of its integral role in mantle circulation, mixing, and evolution.
Dissertation/Thesis
Ph.D. Geological Sciences 2012

The primary focus of this thesis is to examine the detailed seismic structure of the northern Cascadia margin, including the Cascadia basin, the deformation front and the continental shelf. The results of this study are contributing towards understanding sediment deformation and tectonics on this margin. They also have important implications for exploration of hydrocarbons (oil and gas) and natural hazards (submarine landslides, earthquakes, tsunamis, and climate change). The first part of this thesis focuses on the role of gas hydrate in slope failure observed from multibeam bathymetry data on a frontal ridge near the deformation front off Vancouver Island margin using active-source ocean bottom seismometer (OBS) data collected in 2010. Volume estimates (∼ 0.33 km^3) of the slides observed on this margin indicate that these are capable of generating large (∼ 1 − 2 m) tsunamis. Velocity models from travel time inversion of wide angle reflections and refractions recorded on OBSs and vertical incidence single channel seismic (SCS) data were used to estimate gas hydrate concentrations using effective medium modeling. Results indicate a shallow high velocity hydrate layer with a velocity of 2.0 − 2.1 km/s that corresponds to a hydrate concentration of 40% at a depth of 100 m, and a bottom simulating reflector (BSR) at a depth of 265 − 275 m beneath the seafloor (mbsf). These are comparable to drilling results on an adjacent frontal ridge. Margin perpendicular normal faults that extend down to BSR depth were also observed on SCS and bathymetric data, two of which coincide with the sidewalls of the slump indicating that the lateral extent of the slump is controlled by these faults. Analysis of bathymetric data indicates, for the first time, that the glide plane occurs at the same depth as the shallow high velocity layer (100±10 mbsf). In contrast, the glide plane coincides with the depth of the BSR on an adjacent frontal ridge. In either case, our results suggest that the contrast in sediments strengthened by hydrates and overlying or underlying sediments where there is no hydrate is what causing the slope failure on this margin. The second part of this dissertation focuses on obtaining the detailed structure of the Cascadia basin and frontal ridge region using mirror imaging of few widely spaced OBS data. Using only a small airgun source (120 cu. in.), our results indicate structures that were previously not observed on the northern Cascadia margin. Specifically, OBS migration results show dual-vergence structure, which could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Understanding the physical and mechanical properties of the basal layer has important implications for understanding earthquakes on this margin. The OBS migrated image also clearly shows the continuity of reflectors which enabled the identification of thrust faults, and also shows the top of the igneous oceanic crust at 5−6 km beneath the seafloor, which were not possible to identify in single-channel and low-fold multi-channel seismic (MCS) data. The last part of this thesis focuses on obtaining detailed seismic structure of the Vancouver Island continental shelf from MCS data using frequency domain viscoacoustic full waveform inversion, which is first of its kind on this margin. Anelastic velocity and attenuation models, derived in this study to subseafloor depths of ∼ 2 km, are useful in understanding the deformation within the Tofino basin sediments, the nature of basement structures and their relationship with underlying accreted terranes such as the Crescent and the Pacific Rim terranes. Specifically, our results indicate a low-velocity zone (LVZ) with a contrast of 200 m/s within the Tofino basin sediment section at a depth 600 − 1000 mbsf over a lateral distance of 10 km. This LVZ is associated with high attenuation values (0.015 − 0.02) and could be a result of over pressured sediments or lithology changes associated with a high porosity layer in this potential hydrocarbon environment. Shallow high velocities of 4 − 5 km/s are observed in the mid-shelf region at depths > 1.5 km, which is interpreted as the shallowest occurrence of the Eocene volcanic Crescent terrane. The sediment velocities sharply increase about 10 km west of Vancouver Island, which probably corresponds to the underlying transition to the Mesozoic marine sedimentary Pacific Rim terrane. High attenuation values of 0.03 − 0.06 are observed at depths > 1 km, which probably corresponds to increased clay content and the presence of mineralized fluids.
Graduate
0373
0372
0605
subbarao@uvic.ca